Peripheral cannabinoid receptors exert a powerful inhibitory control over pain initiation, but the endogenous cannabinoid signal that normally engages this intrinsic analgesic mechanism is unknown. To address this question, we developed a peripherally restricted inhibitor of fatty acid amide hydrolase (FAAH), the enzyme responsible for the degradation of the endocannabinoid anandamide. The compound, called URB937, suppresses FAAH activity and increases anandamide levels outside the central nervous system (CNS). Despite its inability to access brain and spinal cord, URB937 attenuates behavioral responses indicative of persistent pain in rodent models of inflammation and peripheral nerve injury, and suppresses noxious stimulus-evoked neuronal activation in spinal cord regions implicated in nociceptive processing. CB1 receptor blockade prevents these effects. The results suggest that anandamide-mediated signaling at peripheral CB1 receptors controls the transmission of pain information to the CNS. Brain-impermeant FAAH inhibitors, which strengthen this gating mechanism, offer a new approach to pain therapy.
Anandamide, the naturally occurring amide of arachidonic acid with ethanolamine, meets all key criteria of an endogenous cannabinoid substance (Devane, W. A. et al. Science, 258, 1946-1949 (1992)): it is released upon demand by stimulated neurons (Di Marzo, V. et al., Nature, 372, 686-691 (1994); Giuffrida, A. et al., Nat. Neurosci., 2, 358-363 (1999)); it activates cannabinoid receptors with high affinity (Devane, W. A. et al. Science, 258, 1946-1949 (1992)) and it is rapidly eliminated through a two-step process consisting of carrier-mediated transport followed by intracellular hydrolysis (Di Marzo, V. et al., Nature, 372, 686-691 (1994); Beltramo, M. et al., FEBS Lett., 403, 263-267 (1997)). Anandamide hydrolysis is catalyzed by the enzyme fatty acid amide hydrolase (FAAH), a membrane-bound serine hydrolase (Cravatt, B. F. et al., Nature, 384, 83-87 (1996); Patricelli, M. P. et al., Biochemistry, 38, 9804-9812 (1999)) (WO 98/20119) (U.S. Pat. No. 6,271,015) that also cleaves other bioactive fatty ethanolamides, such as oleoylethanolamide(cis-9-octadecenamide)) (Rodriguez de Fonseca, F. et al. Nature, 414, 209-212 (2001)) and palmitoylethanolamide (Calignano, A. et al., Nature, 394, 277-281 (1998)). Mutant mice lacking the gene encoding for FAAH cannot metabolize anandamide (Cravatt, B. F. et al., Proc. Natl. Acad. Sci. U.S.A., 98, 9371-9376 (2001)) and, though fertile and generally normal, show signs of enhanced anandamide activity at cannabinoid receptors, such as reduced pain sensation (Cravatt, B. F. et al., Proc. Natl. Acad. Sci. U.S.A., 98, 9371-9376 (2001)). This suggests the possibility that drugs targeting FAAH may heighten the tonic actions of anandamide, while possibly avoiding the multiple, often unwanted effects produced by Δ9-THC and other direct-acting cannabinoid agonists (Hall, W., et al., Lancet, 352, 1611-1616 (1998); Chaperon, F., et al., Crit. Rev. Neurobiol., 13, 243-281 (1999)).
Pain perception can be effectively controlled by neurotransmitters that operate within the CNS. This modulation has been well characterized in the dorsal horn of the spinal cord, where impulses carried by nociceptive (pain-sensing) fibers are processed before they are transmitted to the brain. In addition to these central mechanisms, intrinsic control of pain transmission can occur at terminals of afferent nerve fibers outside the CNS. One prominent example of peripheral regulation is provided by the endogenous opioids, which are released from activated immune cells during inflammation and inhibit pain initiation by interacting with opioid receptors localized on sensory nerve endings1,2.
It has been proposed that endocannabinoid mediators might serve an analogous function to that of the opioids, because pharmacological activation of peripheral CB1 and CB2 cannabinoid receptors inhibits pain-related behaviors3-7 while genetic disruption of CB1 receptor expression in primary nociceptive neurons exacerbates such behaviors8. Moreover, there is evidence that clinical conditions associated with neuropathic pain or inflammation, such as complex regional pain syndrome and arthritis, may be accompanied by peripheral elevations in the levels of the endocannabinoid anandamide9,10. Another major endocannabinoid ligand, 2-arachidonoylglycerol (2-AG), has also been implicated in nociceptive signaling outside the CNS8,11.
Much attention has been directed toward the role of anandamide in pain. Methods of treating pain by administering anandamide and palmitoylethanolamide are disclosed in U.S. Patent Application Publication No. 20020173550. Methods of treating pain by administering inhibitors of FAAH are disclosed in U.S. Patent Application Publication Nos. 20040127518 and 20030134894. Methods of treating pain by administering inhibitors of anandamide transport are disclosed in U.S. Patent Application Publication No. 20030149082.
Although these findings suggest that the endocannabinoid system serves an important function in the peripheral regulation of nociception, they offer no definitive insight on the identity of the endogenous ligand, or ligands, involved in this function. Filling this gap is essential, however, to gain a molecular understanding of the intrinsic mechanisms that control pain initiation and to discover new analgesic agents devoid of central side effects. In the present study we identified and characterized a brain-impermeant inhibitor of the anandamide-degrading enzyme, FAAH, with the aim of magnifying the actions of peripheral anandamide and unmasking their possible role in the control of emerging pain signals12. A particular concern in the development and therapeutic use of FAAH inhibitors is their ability to modulate endogenous cannabinoid systems within the CNS system to cause unwanted psychotropic or mood-altering effects.
The present invention addresses these and other needs by providing peripherally restricted FAAH inhibitors and method of their use in the treatment of a variety of conditions, including pain and/or inflammation.